The present invention relates to a fast atom beam source which is capable of emitting a fast atom beam efficiently at a relatively low discharge voltage.
Atoms and molecules have a thermal motion in the atmosphere at room temperature with a kinetic energy of about 0.05 eV. "Fast atoms" are atoms and molecules that have a kinetic energy much larger than 0.05 eV, and when such particles are emitted in one direction, they are called a "fast atom beam".
FIG. 5 shows one example of the structure of a fast atom beam source that emits argon atoms with a kinetic energy of 0.5 to 10 keV, among conventional fast atom beam sources designed to generate fast beams of gas atoms. In the figure, reference numeral 1 denotes a cylindrical cathode, 2 a doughnut-shaped anode, 3 a DC high-voltage power supply (0.5 to 10 kV), 4 a gas nozzle, 5 argon gas, 6 plasma, 7 fast atom emitting holes, and 8 a fast atom beam. The operation of the conventional fast atom beam source is as follows:
The constituent elements, exclusive of the DC high-voltage power supply 3 and a discharge stabilizing resistor (not shown), are incorporated in a vacuum container (not shown). After the vacuum container has been sufficiently evacuated, argon gas 5 is injected into the cylindrical cathode 1 from the gas nozzle 4. Meanwhile, a DC voltage is impressed between the anode 2 and the cathode 1 from the DC high-voltage power supply 3 in such a manner that the anode 2 has a positive potential, and the cathode 1 a negative potential. Consequently, electric discharge occurs between the cathode 1 and the anode 2 to generate plasma 6, thus producing argon ions and electrons. During this process, electrons that are emitted from one end face of the cylindrical cathode 1 are accelerated toward the anode 2 and pass through the central hole in the anode 2 to reach the other end face of the cathode 1. The electrons reaching the second end face lose their speed. Then, the electrons are turned around and are accelerated toward the anode 2 to pass again through the central hole of the anode 2 before reaching the first end face of the cathode 1. Such repeated motion of electrons forms a high-frequency vibration between the two end faces of the cylindrical cathode 1 across the anode 2, and while undergoing the repeated motion, the electrons collide with the argon gas to produce a large number of argon ions.
The argon ions produced in this way are accelerated toward each end face of the cylindrical cathode 1 to obtain a sufficiently large kinetic energy. The kinetic energy obtained at this time is, for example, about 1 keV when the discharge sustaining voltage impressed between the anode 2 and the cathode 1 is 1 kV. There is a turn point of electrons vibrating at high frequency in the vicinity of each end face la of the cylindrical cathode 1. This point is a space where a large number of electrons with low energy are present. Argon ions change to argon atoms in this space by colliding and recombining with the electrons. In the collision between the ions and the electrons, since the mass of the electrons is so much smaller than that of the argon ions that their mass can be ignored, the argon ions deliver the kinetic energy to the atoms without substantial loss, thus forming fast atoms. Accordingly, the kinetic energy of the fast atoms is about 1 keV. The fast atoms accelerated are emitted in the form of a fast atom beam 8 to the outside through the emitting holes 7 provided in one end face 1a of the cylindrical cathode 1.
The above-described conventional fast atom beam source suffers, however, from some problems described below. To increase the rate of emission of the fast atom beam, the prior art needs to raise the discharge voltage, or use a magnet jointly with the described arrangement, or increase the pressure of the gas introduced and cannot adopt any other method that does not result in an increase in the energy of the fast atom beam, or an increase in the overall size of the apparatus, or an extension in the energy band of the fast atom beam, etc. Thus, the prior art presents many problems and difficulties during use.